sort in hashmap

2020-09-18

##Hashmap in java, specifiy the Comparator

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class ComparatorImpl implements Comparator<Map.Entry<Character,Integer>>{
	public int compare(Map.Entry<Character,Integer> a ,Map.Entry<Character,Integer> b){
		return b.getValue() - a.getValue();
	}
}

   public String frequencySort(String s) {
       StringBuilder ret = new StringBuilder();
       HashMap<Character, Integer> map = new HashMap<>();

       for(int i = 0; i < s.length(); i++){
           map.put(s.charAt(i),  map.getOrDefault(s.charAt(i),0) + 1);
       }

       ArrayList<Map.Entry<Character,Integer>> list = new ArrayList<>(map.entrySet());
       Collections.sort(list, new ComparatorImpl());

       for(Map.Entry<Character,Integer> c: list){
           for(int i =0; i < c.getValue(); i++){
               ret.append(c.getKey());
           }
       }
       return ret.toString();
   }

This is to define the sort algorithm,

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class ComparatorImpl implements Comparator<Map.Entry<Character,Integer>>{
	public int compare(Map.Entry<Character,Integer> a ,Map.Entry<Character,Integer> b){
		return b.getValue() - a.getValue();
	}
}

1 convert the hash Entry to array list, and sort.
2 there is other way, such as TreeSet

Reference:
https://www.baeldung.com/java-hashmap-sort